The following disclosure is based on German Patent Application No. 101 04 177.2 filed on Jan. 24, 2001, which is incorporated into this application by reference.
1. Field of the Invention
This invention relates to a catadioptric projection lens for projecting a pattern from an object plane onto an image plane.
2. Description of the Related Art
Such projection lenses are used in projection exposure systems for producing semiconductor devices and other microdevices, in particular in wafer scanners and wafer steppers. They are used to project patterns of photo masks or reticle plates (in the following simply called masks or reticles) onto an object coated with a photosensitive layer. The projection is performed with highest resolution and in reduced scale.
In order to create increasingly fine structures, it is necessary to increase the numerical aperture (NA) of the projection lens on one hand and to use increasingly shorter wavelengths on the other hand, preferably ultraviolet light with wavelengths of less than approx. 260 nm.
In this wavelength range there are only few sufficiently transparent materials for producing the optical components, in particular synthetic quartz glass and fluoride crystals, such as calcium fluoride, magnesium fluoride, lithium calcium aluminum fluoride, lithium strontium aluminum fluoride, barium fluoride, lithium fluoride, or the like. Since the Abbxc3xa9 constants of the available materials are relatively close together, it is difficult to provide pure refractive systems with sufficient correction of color aberrations (chromatic aberrations). In principle, this problem could be solved by using pure reflective systems. However, the fabrication of such mirror systems is costly.
Considering the problems mentioned above, catadioptric systems are preferable for projection lenses of very high resolution. In catadioptric systems refracting and reflecting components, therefore in particular lenses and mirrors, are combined.
When using mirror surfaces for projection, it is advantageous to use beam splitters to achieve obscuration-free and vignette-free images. There exist systems with geometrical beam splitters as well with physical beam splitters. A system with a geometrical beam splitter that uses two deviating mirrors is shown in EP 0 989 434 (corresponding to the U.S. Ser. No. 09/364382). Systems with a geometrical beam splitter have the disadvantage that they must necessarily be off-axis systems. By using a physical beam splitter, however, on-axis systems can be realized.
A system with a physical beam splitter and an intermediate image is known from EP-A-0 475 020 (corresponding to U.S. Pat. No. 5,052,763). This system has at least one catadioptric entry system and one dioptric exit system. The mask to be projected rests directly on a beam splitter, designed as a beam splitter cube (BSC). With the help of the beam splitter, part of the light reflected by the catadioptric system is diverted to the dioptric system. With the object to be projected resting directly on the beam splitter, the correction possibilities of the total system are restricted. Furthermore, this contact procedure has extremely high demands with respect to material quality and can cause mechanical problems due to the lack of working distance.
From EP-A-0 350 955 (corresponding to U.S. Pat. No. 4,953,960), a catadioptric projection lens without intermediate image is known. This projection lens system consists of a first lens group between the object plane and a physical beam splitter, a second lens group between the physical beam splitter and a concave mirror, and a third lens group between the physical beam splitter and the image plane. The lens group between the beam splitter and the concave mirror is supposed to correct comas of low degrees, spherical aberrations of the mirror, and the Gauss"" error.
From DE-A-42 03 464 (corresponding to U.S. Pat. No. 5,402,267), a catadioptric projection lens with physical beam splitter and without intermediate image is known that permits high rear numerical aperture of at least 0.5 with a favorable construction and low adjustment sensitivity. The system distinguishes itself mainly by the fact that there is no lens group between the concave mirror and the beam splitter and that the concave mirror has a considerable reduction effect, i.e. a strongly reducing magnification. The correction of the chromatic longitudinal ray aberration (CHL) is mainly achieved with a strongly convergent ray trajectory in the beam splitter cube and may cause total achromatization of the chromatic longitudinal ray aberration. Typically the ray trajectory in front of the mirror, i.e. in the first passage through the beam splitter, is nearly collimated, while the ray trajectory behind the mirror, i.e. in the second passage through the beam splitter is normally strongly convergent. The system aperture is preferably located where the concave mirror is and is defined by the mirror rim. The aperture may also be defined on the mirror-side bounding surface of the beam splitter or between mirror and beam splitter. The strongly convergent ray trajectory after the concave mirror has the further advantage that only little positive focal power is needed after the beam splitter and that the beam heights are relatively small in this area so that negative effects on the chromatic aberration due to large beam heights in this area can be avoided. Projection lenses with these or comparable constructional and functional characteristics are called type I for the purpose of this application.
With these advantages, type I lenses have the disadvantage that the radiation reaches the beam splitter surface convergent, in particular in the second passage after being reflected by the concave mirror, causing a very large angle of incidence range. This has higher demands with respect to the quality of the beam splitter layer. In addition, the strong convergence of the ray trajectory after the concave mirror leaves very little room for lenses behind the beam splitter and thus little room for correctional measures. A further increase of the rear numerical aperture would require enlarging the beam splitter cube so that the image plane would have to be even closer to the beam splitter. For this reason, projection lenses of type I are also known as aperture limited.
Essentially similar problems also occur for other projection lenses that are constructed according to type I as far as the build-up and the ray trajectory is concerned. Among these are the projection lenses shown in the US patents U.S. Pat. Nos. 6,118,596, 6,108,140, 6,101,047. Large angles of incidences on the beam splitter surfaces may also occur in systems that create an intermediate image, shown for example in U.S. Pat. No. 5,808,805 or U.S. Pat. No. 5,999,333.
From U.S. Pat. No. 5,771,125, a catadioptric projection system with physical beam splitter and without an intermediate image is known where the rays are slightly divergent during their first passage through the beam splitter layer and are collimated during the second passage after being reflected by the concave mirror. This is to avoid deterioration of the image quality due to the dependence of the beam splitter layer""s reflectivity on the angle of incidence. The collimation of the reflected light is achieved by keeping the focal power of the mirror group containing the concave mirror relatively low. In the system of EP-A-0 602 923 (corresponding to U.S. Pat. No. 5,715,084), however, a positive lens is provided in front of the physical beam splitter in order to collimate the rays that reach the beam splitter layer during the first passage. After being reflected by the concave mirror, it is convergent.
In order to minimize the angle of incidence on the beam splitter layer, DE-A-44 17 489 (corresponding to U.S. Pat. No. 5,742,436) suggests positioning at least one convergent lens on the object-side in front of the physical beam splitter in a catadioptric projection system without intermediate image in order to make the ray arriving at the beam splitter layer parallel. Behind the physical beam splitter in the catadioptric lens part, i.e., between the beam splitter and the concave mirror, a dispersing lens group with a negative lens is provided, compensating for the effect of the convergent lens in front of the beam splitter and correcting the chromatic longitudinal ray aberration. In this design, called type II for the purpose of this application, the system aperture is behind the beam splitter cube and the ray trajectory in the beam splitter cube is largely collimated in both passage directions.
As the ray trajectory in the beam splitter cube is largely collimated in both passage directions, problems occurring with large ranges of angles of incidence are avoided. Another advantage of the collimated ray trajectory in the second passage after the reflection at the concave mirror is that on the rear of the beam splitter in the aperture space, there is enough room for building corrective measures. A disadvantage of the arrangement according to DE-A-44 17 489 is that the correction of the longitudinal chromatic aberration (CHL) is incomplete.
One object of the invention is to provide a projection lens with a physical beam splitter that avoids the disadvantages of the prior art. It is another object to allow for a nearly complete chromatic correction for very large image side numerical aperture and favorable construction. This shall be achieved through low material use with respect to mass and/or number of optical components.
As a solution to this and other objects, this invention, according to one formulation, provides a catadioptric projection lens that projects a pattern from the object plane onto an image plane which includes the following components between object plane and image plane in the given order: a first lens part for creating a ray directed at a physical beam splitter, a physical beam splitter with a beam splitter surface, a mirror group with a concave mirror and a mirror focal power of the mirror group, and a second lens part with positive focal power for creating an image of the pattern on the image plane, wherein the focal power of the mirror group is large and the system aperture is located imagewise behind the concave mirror.
The large focal power of the mirror group permits a ray trajectory in which the light arrives at the mirror group divergent and exits convergent in direction of the beam splitter surface. It is thus possible for the beam splitter to be radiated with non-collimated light in the first as well as in the second passage. Due to the angled light passage, the beam splitter can thus contribute to the chromatic correction. It is advantageous if the sum of the absolute values of the paraxial peripheral ray angles is much larger than zero during the first and the second passage. The sum of the absolute values of the paraxial peripheral ray angles is preferably more than 30% or 40% of the numerical aperture, in particular, it may be in the range around 60% of the numerical aperture. At the same time, the positioning of the system aperture imagewise behind the concave mirror ensures that, even for high image side numerical aperture, the maximum beam heights on the rear of the beam splitter are limited to acceptable values. This avoids problems due to large lens diameters in the construction of the second lens part. The system aperture here is the axial position where the principal ray intersects with the optical axis. A position of the system aperture between the beam splitter surface and the image plane, in particular a position near or at the rear exit surface of the beam splitter, is preferable.
The projection preferably takes place without an intermediate image.
The first lens part is preferably designed so that a divergent ray is created that is directed at the beam splitter. Strongly diverging rays are preferable where peripheral ray angles may occur that are larger than 20% of the rear numerical aperture and may be between 30% and 40% of the rear numerical aperture for example. The peripheral ray angle u in this case is the product of the refraction coefficient n of the medium that is passed and the sine of the angle between peripheral ray and optical axis.
In preferred embodiments, a strong beam divergence with favorable lens part diameter is achieved by having a first lens part in front of the beam splitter that has a negative focal power, preferably at least two negative lenses, to create a narrow section in the ray trajectory.
It is also favorable to have a focal power of the mirror group that is so large that the ray on the rear of the mirror group is convergent. To achieve a considerable contribution to the chromatic correction, is has been shown to be effective to have a peripheral ray angle of the exiting convergent ray that is larger than approx. 10% of the rear numerical aperture of the projection lens. On the other hand, strongly convergent rays should be avoided so that there is enough space behind the beam splitter to set up lenses and other optical components affecting the rays. For this reason, the peripheral ray angles should not exceed or not considerably exceed approx. 30% of the rear numerical aperture.
It is effective to have an absolute value of the peripheral ray angle during the first passage before passing through the mirror group that is larger than the angle after the mirror reflection. Preferably, a strong divergence before passing through the mirror group is turned into a convergence with a lower absolute value so that the main contribution to the chromatic correction already takes place during the first passage through the beam splitter.
Particularly preferable are embodiments where there is no free-standing lens between the beam splitter surface and the concave mirror. The concave mirror is preferred to have positive magnification. With its large focal power it contributes to the total focal power without causing chromatic aberrations. It also corrects the Petzval sum of the projection lens.
With the invention, it is possible to perform the chromatic correction mainly before the ray enters the second lens part, i.e., mainly in the area of beam splitter and mirror group. This makes it possible to have an especially favorable design of the second lens part with respect to dimensioning and material requirements. In particular, it is unnecessary to provide highly effective chromatic corrective means so that the second lens part may be made of lenses made of a single material, at least in the area of large beam heights, i.e., at a certain distance from the image plane. Embodiments of the invention distinguish themselves in that essentially all transparent optical components are made of one material, for example calcium fluoride, also known as fluorspar (CaF2), or synthetic quartz glass (SiO2). When using SiO2, a second radiation resistant material, such as CaF2, may be used close to the image plane where high power-densities of the rays occur in order to avoid compaction problems for example. The lens material used mainly or exclusively can thus be calcium fluoride or another fluoride crystal material, in particular if the system is designed for wavelengths of less than approx. 160 nm. For larger wavelengths, such as wavelengths around 193 nm, it is also possible to use synthetic quartz glass mainly or exclusively.
In order to achieve good monochromatic correction or a high imaging power and low aberration with very high numerical aperture while reducing material expenditure, one or more aspherical surfaces may be used in an embodiment. In this case, a larger number of aspherical surfaces is normally used but preferably not more than eight. In particular regarding the correction of the spherical aberration and of comas, it is effective to position at least one aspherical surface in the area of the system aperture. A particularly effective correction is achieved if the ratio h/xcfx86 between the peripheral ray height h on the surface and the radius xcfx86 of the opening of the system aperture is between approx. 0.8 and approx. 1.2. The peripheral ray height close to the aspherical surface should thus be close to the maximum peripheral ray height.
In order to enable an effective correction of the distortion and other field aberrations, it is effective to also provide at least one aspherical surface close to the field, i.e., close to the reticle, or on the object plane and/or close to the wafer or the image plane. These areas in proximity to the field distinguish themselves by the fact that the ratio h/xcfx86 is smaller than approx. 0.8. It is favorable to have at least one aspherical surface in proximity to the field and at least one aspherical surface close to the system aperture. It is thus possible to provide sufficient correction for all optical aberrations discussed.
The previous and other properties can be seen not only in the claims but also in the description and the drawings, wherein individual characteristics may be used either alone or in sub-combinations as an embodiment of the invention and in other areas and may individually represent advantageous and patentable embodiments.